You open a bag of potatoes after two weeks of storage and are hit by a pungent smell, and at the bottom you find tubers that have turned into a watery, creamy paste that runs onto what is around it. Or you pick a head of cabbage that looked healthy, and its heart softens and blackens within days. This is bacterial soft rot, the best-known disease that costs growers and traders a crop that was ready for sale, because it does not stop at the field but follows the crop into the crates, the cold room, and the shop shelves.
The news you need to know from the start: there is no pesticide that cures a tuber or fruit that has started rotting from this bacterium. The battle is won by controlling three things: wounds, free water on the surface of the crop, and temperature. In this article we explain how to recognize the disease, why it explodes in storage, and what to do from the field to storage.
Disease: A soft rot caused by bacteria that turn the tissue of tubers and fruit into a slimy, watery, foul-smelling mass.
Timing: It usually starts in storage, or in the field after wounds, high humidity, and water pooling around the tubers.
Speed: A crate of potatoes can turn into a loss within days if one infected tuber is left among healthy ones.
Common mistake: Storing wounded or wet tubers together, because wounds are the entry door for the bacteria.
Rule: Drying the tubers, preventing wounds, and ventilating the store are the most successful prevention, and there is no sure cure for a rotting tuber.
What Causes Soft Rot?
The main cause is the bacterium Pectobacterium carotovorum (formerly called Erwinia carotovora), together with close relatives: Pectobacterium atrosepticum, which causes blackleg in potatoes, and Dickeya species, which are more active in high heat. University of Wisconsin research indicates that Dickeya prefers temperatures above 25 degrees Celsius (77 F) and its damage intensifies above 27 degrees Celsius (81 F), while Pectobacterium dominates below 25 degrees Celsius.
These bacteria do not need any special strength to break into the plant. They are already present in soil, irrigation water, and on the surface of many healthy tubers, waiting for an opportunity. Their weapon is enzymes that dissolve pectin, the "cement" that binds cell walls together. When it dissolves, the cells separate and the tissue collapses into a soft, watery mass.
That is why soft rot affects a long list of crops: potatoes, carrots, onions, cabbage, broccoli, lettuce, peppers, tomatoes, cucumbers, melons, and even ornamental bulbs.
How to Recognize It: Texture, Smell, and the Boundary Line
Soft rot is recognized by touch before sight: soft, watery tissue that gives under the finger, a dark line separating it from the healthy tissue, and then a foul smell that comes later. Its look differs a little by crop.
In Potato: Tubers and Blackleg
- Starting point: It usually starts at the lenticels (the small pores in the skin), at a wound, or at the end where the tuber attached to the stolon, as small, sunken, water-soaked spots.
- Tissue texture: The infected tissue is creamy to light brown, soft and watery, gives under the finger, and may ooze liquid.
- The boundary line: A clear dark brown or black line separates the rotten tissue from the healthy.
- Smell: Mild at first, then very foul once secondary organisms invade the decomposing tissue. A pungent smell is a sign that the rot has advanced, not its first symptom.
- Deceptive skin: The skin often looks intact while the inside is decomposed, so the tuber bursts when pressed.
Blackleg in the field: Stunted plants standing stiffly upright, with yellowing leaves whose edges curl upward, then wilting. At the base of the stem there is a glossy, ink-black rot that starts from the planted mother tuber, and the stem may pull easily out of the soil.
In dry soil the blackening may be limited to the pith of the stem. The important difference: ordinary soft rot may also cause wilting, but without this inky black rot at the base.
In Other Vegetables
- Cabbage, lettuce, and broccoli: A slimy rot that starts at a broken leaf or the cut base and spreads to the heart, with a foul smell.
- Onion: The inner scales become watery and soft, and if you press the neck, a foul-smelling liquid comes out, while the onion looks normal from the outside.
- Carrot: A watery rot that starts at the tip or at a crack in the root, common in washed carrots stored wet.
- Pepper and tomato: The fruit collapses into a bag of water under a thin skin, usually starting from an insect wound or the stem-end area, or after washing in contaminated water.
- Cucumber and melon: Soft, watery spots that start where the fruit touches wet soil or at a wound.
Do Not Confuse It With
- Fusarium dry rot in potato: A dry, wrinkled rot with rings and an internal cavity with white or pink fungal growth. It often opens the way for soft rot, so they occur together.
- Late blight tuber rot: Reddish-brown, granular, firm tissue under the skin, not soft and watery, unless soft rot bacteria invade it later.
- Pink rot (from oomycetes): The cut tissue turns pink after about fifteen to thirty minutes of exposure to air, and is rubbery, not pasty.
- Cold and freezing: Frozen tubers suddenly turn watery after thawing, and soft rot follows later as a secondary infection.
Why Does the Disease Explode in Storage?
The bacteria need only three things: an open door, free water, and oxygen shortage. All of them come together easily after harvest:
- Wounds and bruises: Every drop from a height, and every scratch from harvesting machinery or a crate edge, is an open door. Immature tubers with soft skin wound easily.
- A film of water on the surface: Free water on the skin cuts off oxygen to the lenticels, so they swell and open, and the way is open to the bacteria. This is the most dangerous point of all.
- Condensation: Moving warm produce from the field into a cold room, or covering piles with plastic, makes water vapor condense on the surface. The result is the same deadly water film without you watering anything.
- Heat: University of California guidance indicates that a tuber pulp temperature above about 21 degrees Celsius (70 F) at harvest and afterward is among the factors that promote soft rot.
- Chain infection: One rotting tuber drips its bacteria-laden liquid onto the tubers beneath it, so a small pocket becomes a whole pile within days. That is why rot usually starts at the bottom of the bag or the middle of the pile, where heat and humidity are higher and ventilation is weaker.
The Practical Plan: From Field to Storage
Because there is no cure, protection is built on a chain of connected steps: healthy seed, irrigation, and nutrition in the field, then harvest without wounds, then curing and storage without wetness. Any weak link in this chain opens the door to rot.
In the Field Before Harvest
- Healthy seed: Blackleg almost always starts from the mother tuber. Plant certified seed, and if you cut seed pieces, leave them until the cut surface heals and forms a skin before planting, or plant small whole tubers.
- Avoid saturated soil: Long, infrequent irrigation is better than short, frequent irrigation, morning irrigation is better, and the low parts of the field where water stands are the first to be infected.
- Calcium: University of California guidance indicates that adding soluble calcium in batches during tuber bulking reduces the incidence and severity of soft rot, because calcium strengthens the bonds between the cell walls that the bacterial enzymes attack.
- Insects: Insects that gnaw tubers and fruit make wounds and carry bacteria from plant to plant, so controlling them is part of controlling rot.
- Crop rotation: Rotate with corn, wheat, or alfalfa, and get rid of volunteer potatoes and piles of rejected tubers near the field.
At Harvest and Preparing Tubers for Storage
- Wait for skin set: Harvest after the skin has fully hardened. In potatoes, it is advised to wait about three weeks after vine kill.
- Choose the harvest time: Avoid harvesting when the soil is wet and its mud sticks to the tubers, and harvest in the cool hours of the day so that tubers go in with a lower pulp temperature.
- Reduce impacts: Reduce the drop height at every transfer point, and do not throw crates.
- Separate the infected: Do not harvest the infected part of the field together with the healthy part, and if you must, store it separately and market it first.
Curing before cooling: Leave potatoes in a shaded, well-ventilated place at about 10 to 13 degrees Celsius (50 to 55 F) for 10 to 14 days, with high air humidity but no wetness on the surface. During this period wounds heal and a corky layer forms that closes the doors to bacteria.
After that, lower the temperature gradually for storage. Low temperature slows the bacteria, but very low temperature turns starch into sugar in table and processing potatoes, so set it according to the use of the crop.
In the Store
- Ventilation before everything: Air moving between crates and from beneath the pile, not a heap stacked on a solid floor.
- Do not wash potatoes meant for long storage: If washing is necessary before marketing, use clean water treated with chlorine, then dry them completely before packing. University of California guidance notes keeping recycled water in wash lines treated with free chlorine between 50 and 200 parts per million.
- Inspect the store with your nose and hands: Any strange smell or warm spot in the pile means a pocket of rot. Remove it immediately along with the tubers next to it.
- Do not mix new with old: And do not put the crop from the wet parts of the field with the rest.
- Clean and disinfect the store, crates, and belts: Before receiving the new crop, because the bacteria survive in dried rot residues on wood and plastic.
Pepper, Tomato, and Leafy Vegetables After Picking
- Wash water temperature: If you wash fruit in a tank, keep the water about 5 degrees Celsius (9 F) warmer than the fruit pulp. Water colder than the fruit makes the air inside it contract and pulls contaminated water in through the stem end and wounds.
- Tank cleanliness: Treat the tank water with chlorine and renew it constantly, because a contaminated wash tank can spread rot through a whole shipment.
- Rapid cooling: Cool the crop quickly after picking, and store it clean and dry. The University of Delaware recommends keeping vegetables after harvest between about 4 and 7 degrees Celsius (39 to 45 F) to limit the spread of rot.
- Dry packing: Do not pack lettuce and cabbage wet in closed bags.
Is There a Pesticide for Soft Rot?
Frankly: there is no chemical treatment that stops the rot inside a tuber or fruit that has been infected. Fungicides do not kill this bacterium, and copper compounds may reduce bacteria on leaf surfaces in some crops, but they do not reach the inside and do not protect a stored crop.
Agricultural antibiotics are banned or restricted in many countries, and are not a storage solution anyway. So at Orkida we build protection on prevention, and we offer from the store what really serves it:
- Virkon S (5 kg): from LANXESS, potassium peroxymonosulfate, a broad-spectrum oxidizing disinfectant that kills bacteria, fungi, and viruses on surfaces. It is not sprayed on the crop. We recommend it for disinfecting crates, belts, store floors and walls, tools, and empty wash tanks before receiving a new crop, at the concentration written on the label, after removing dirt and rot residues first, because organic matter uses up the disinfectant. It is also available in a Virkon S 10 kg pack for stores and large farms.
- Mainstay Calcio: from Cosmocel, a nitrate-free liquid calcium fertilizer added with drip irrigation or center-pivot spraying. We prefer it to support potatoes and vegetables during tuber and fruit bulking, because calcium strengthens cell walls and improves fruit firmness and storage ability. It is not a pesticide, but it makes tissues less willing to give in to the bacterial enzymes. Apply it in batches at the dose written on the label.
With any product you use, following the concentration written on the label and the pre-harvest interval is what protects the crop and whoever eats it at the same time.
You Found Rot in Storage: What Do You Do Now?
Soft rot multiplies fast in warmth and moisture, so what you do in the first hours decides whether you lose a few tubers or the whole pile. Follow these steps in order:
- Sort immediately: Remove the rotten tubers and everything contaminated by their liquid, and do not postpone to tomorrow, because rot multiplies quickly in warmth.
- Dry what remains: If the healthy tubers are wet with rot liquid or condensation, spread them in a shaded, well-ventilated place until completely dry.
- Increase ventilation and adjust temperature: The goal is to remove any wetness on the surface and lower the temperature of the pile.
- Market the doubtful crop first: And do not try to store it for long.
- Dispose of the rejects far away: From the field and irrigation sources, and do not dump them in a pile near the store.
- After emptying: Clean and disinfect the store and crates before next season.
If the problem is in the field before harvest, review where water collects, because soft rot often occurs together with root rot in poorly drained soil, or enters tissue weakened by fungal diseases such as gray mold.
Frequently Asked Questions About Bacterial Soft Rot
Are potatoes with soft rot safe to eat?
Soft rot bacteria make plants sick, not people, but a rotting tuber has usually been invaded by secondary organisms and lost its quality. If the spot is small and the tuber is firm, you can cut out the affected part with a wide healthy margin. A soft or smelly tuber belongs in the trash.
Why does a rotting potato give off such a foul smell?
The soft rot bacteria themselves do not give a strong smell at first. The pungent smell comes from secondary organisms feeding on the decomposed tissue, so when you smell it the rot has advanced and you should sort immediately.
Is cooling alone enough to prevent rot?
No. Cooling slows the bacteria but does not protect wounded or wet tubers, and may make the problem worse if you move a warm crop into a cold room so that water condenses on its surface. Curing first to heal wounds, then gradual cooling with ventilation.
What is the difference between soft rot and blackleg?
The cause is from the same bacterial family. Blackleg is a field disease that starts from the planted mother tuber and gives an inky black rot at the base of the stem, while soft rot is the name for the decay of the fleshy tissue itself in the field or in storage.
Does spraying a fungicide on potatoes before storage help?
A fungicide does not kill soft rot bacteria, and adding a water spray to tubers without sufficient ventilation may create the water film the bacteria need. If a treatment is needed for fungal diseases, make the spray very light so that the tubers stay almost dry.
Can I plant seed from tubers that showed soft rot?
We do not recommend it. The mother tuber is the main source of blackleg, and seed from a pile where rot appeared often carries the bacteria even if it looks healthy.
You will find the rest of the crop diseases and pests in the Pest Library.